This invention relates to electrical power distribution equipment and in particular to a new and improved electrical distribution transformer especially suited for non-linear load applications.
Non-linear loads are characterized by devices such as switching mode power supplies, electronic ballasts and the like which produce harmonic currents and voltages that affect the distribution transformer. These harmonics are especially rich in the third harmonic and its multiples (triplen). In standard three phase delta/wye (primary winding connected in the delta configuration and secondary winding in the wye configuration), the triplen currents flowing in the wye secondary winding are in phase and additive in the common or neutral line. As a result, the net effect is a rather large triplen current in the neutral line which by transformer action results in a large triplen current that circulates in the primary delta winding. This leads to overheating, equipment breakdown and in the extreme, to fire.
In addition to harmonics produced by a non-linear load, the distribution transformer is also subjected to harmonics on the source bus that feeds its primary winding. Source harmonics can be caused, for example, by other distribution transformers connected in the power distribution system and which are not designed for either elimination of, or substantial attenuation of harmonics produced by their respective loads.
In the past, several attempts have been proposed or made to address the non-linear load problem. One such attempt involves the use of larger transformers to handle the extra heat generated by the triplen currents. Unfortunately, the neutral third harmonic currents have been so high that unacceptable overheating still occurs due to the triplen flux resulting in failure.
Another prior attempt uses filters which are tuned to the third, fifth, seventh and/or ninth harmonics, for example, and are located in the load circuit to block or trap the load produced harmonics. However, the loads are dynamic in operation such that the system resonance value is continually varying. This degrades or even eliminates the effectiveness of the filters.
Still another attempt is the Dry-type Transformer for Non-linear (Non-sinusoidal) Loads of International Transformer Corporation, Montebello, Calif. This transformer employs a number of features. First, it uses an enlarged primary winding conductor to reduce the harmonic (triplen) circulating current heating effect within the primary delta winding. Second, the unit is designed with a lower magnetic flux density in the core so as to handle the additional flux produced by the harmonic currents without saturation of the core. Third, an electrostatic shield is positioned between the windings for attenuation of common mode, high frequency harmonics. Fourth, the secondary winding employs, where necessary, small multiple conductors in parallel to reduce the skin effect of the high frequency harmonics. Fifth, all conductors are insulated and transposed, when necessary, to reduce the stray loss heating effect of the harmonic currents within the windings of the transformer, keeping the temperature rise of the unit within its design limits. Sixth, the neutral terminal has twice the current carrying capacity of the phase currents. Despite all of these features, this transformer is still subject to additive triplen harmonic currents that circulate in its delta winding and non-triplen currents that produce voltage stress within the coils.